In the search for extraterrestrial intelligence (SETI), we are fundamentally looking for signals that have never been detected before. That said, in radio SETI, we do make the assumption that intelligent civilizations emit radio waves, intentionally or not, as a normal byproduct of technology, just like our own civilization. We regularly detect human-generated radio emission (RFI) with our telescopes, and this becomes a very difficult confounding factor for SETI.

In order to find and isolate candidate signals, we need to have algorithms that detect radio signals above an intensity threshold and reliably differentiate them from RFI. As our target signals have technically never been found, we rely on basic assumptions about ET signals — for example, a technosignature should change in frequency smoothly over time as a result of orbital acceleration around its star. However, without example data, it can be hard to assess the effectiveness of our search algorithms and to develop new ones.

To address this, we developed Setigen (https://github.com/bbrzycki/setigen), a Python library to create synthetic technosignatures in radio time-frequency data. Setigen facilitates the creation of fully synthetic radio data, as well as the injection of synthetic signals into real observational data. Signals can be customized along multiple axes, such as signal path in time-frequency space, intensity variation as a function of time, and spectral shape.

Synthetic signals injected into a waterfall plot using setigen.

Setigen is already being used to test Breakthrough Listen signal search methods such as TurboSETI. To do so, we create datasets of synthetic signals at various signal-to-noise levels and evaluate the signal recovery rate of our search algorithms. In addition, Setigen has been used to create hybrid datasets with real data and injected synthetic signals for training machine learning models, including Breakthrough Listen's Kaggle competition (https://www.kaggle.com/c/seti-breakthrough-listen).

More details about the library and the creation of radio SETI data products can be found in Brzycki et al. 2022 (https://arxiv.org/abs/2203.09668), which has been accepted for publication in the Astronomical Journal.

Header image: The carrier signal from the Voyager spacecraft (left) and a synthetic signal from setigen (right).

Ideally, in the search for extraterrestrial intelligence (SETI), we would want to observe every object in the night sky at all times over all frequencies. Yet, as should be obvious, this multidimensional observing strategy is hard given the practical limitations of telescope technology. One strategy to resolve this is strategic target selection, which can narrow down the number of targets and time of observation based on SETI theory. The range of frequency of the observation is dependent on the receivers available on each telescope. Targeted searches can increase the chance of detecting extraterrestrial intelligence (ETI) by focusing on a narrow list of plausible sources to search for signals from technologically advanced extraterrestrial life, or technosignatures. However, since targeted searches rely on the assumption that SETI theory is correct. In case this is incorrect, it is also important to perform broader searches.

One of the most common techniques used to detect exoplanets, as used by TESS, is the transit method. When an exoplanet transits, it crosses between the Earth and the exoplanet's host star. The transit method looks for small dips in the brightness of the host star as the exoplanet blocks some of the star's light. This transit usually only lasts for a small fraction of the exoplanet’s orbit and works best for large exoplanets (larger exoplanets block more light resulting in a larger dip in brightness). Also, for SETI searches, during a transit any ETI signal directed away from the host star will appear brightest to an observer on Earth!

Strategically targeting TESS TOIs is important for SETI searches because of the geometrical alignment of Earth with the extraterrestrial system. Since TESS TOIs are transiting, as observed from Earth, ETI may be more likely to transmit a signal to those that can observe it transiting. This is because ETI may understand that transiting exoplanet systems are some of the most detectable and assume that those that can see their system transit would observe them for technosignatures.

Recently, as part of Breakthrough Listen (BL), Traas et al. (2021) (see the paper here and blog post here) performed a search of 28 Transiting Exoplanet Survey Satellite (TESS) Targets of Interest (TOIs), using four of the receivers (L, S, C, and X, spanning radio frequencies from 1 - 11 GHz) available at the Green Bank Telescope (GBT). Building off of Traas et al. (2021), this past summer I performed a technosignature search of targets that transit during their GBT observations. Traas et al. selected any TESS TOIs observed with all four receivers by BL. Instead, during my project we used data from TESS TOIs that were in transit during the BL GBT observation, further narrowing the targeting strategy of Traas et al. As mentioned above, by doing so, we are increasing the likelihood of detecting signals directed in the opposite direction to the planet’s host star.

In the end, we did not detect any evidence of extraterrestrial life in the systems we observed. However, we were the first to place constraints on technosignatures from TESS TOIs observed during transit. More information on our target selection, analysis, and results can be found in Franz et al. (2022), which was recently accepted for publication in The Astronomical Journal.

Main Image Credit: NASA

My name is John Hoang, postdoctoral scholar working at the Berkeley SETI Research Center (BSRC). With help from the team, I recently organized a 2-day workshop to inform the community about several current results and upcoming OSETI activities, as well as to open opportunities for wider involvement. The workshop was thematically divided into 2 days:

Day 1 - State of the art: After opening remarks from BSRC’s Director and the Breakthrough Prize Foundation’s Chairman, the workshop commenced with a talk on the theory behind OSETI observations. Subsequent talks included the latest results from various experiments, followed by updates from instruments capable of OSETI techniques.

Day 2 - State of the future and making it happen: The second day began with a series of talks about upcoming facilities capable of large-scale optical surveys, followed by student engagement and collaborating opportunities at UC Berkeley and beyond. The final part of the workshop was dedicated to open discussions from the workshop’s participants.

For those who couldn’t join the workshop, or only attended partially, we got you covered. The talks and discussions are now publicly available on Youtube.

So where do we go from here? A lot of exciting ideas were generated during this workshop, and many new projects began to take shape. I am also summarizing the workshop's contents into a white paper. In the longer term, there could be a platform to coordinate concurrent observation between telescopes, or a notification system similar to the existing ones such as ATel or GCN to encourage follow-up observation on certain targets of interest. And finally we can perhaps start thinking about organizing similar workshops on a more regular basis.

Image: Pexels / Dick Hoskins

The future of interstellar travel may be the most important factor when considering SETI strategy. Are societies spanning entire galaxies, with billions of inhabited worlds, likely to exist? Are they even possible? Two new papers that I have submitted consider this issue in very different ways.

Interstellar travel has many difficulties, especially if we want to go beyond quick flybys to actually voyaging to nearby stars with a crew. One of them is the sheer distance between the stars. It would take decades or centuries to reach our nearest star, Proxima Centauri. Another is the stuff in between the stars, especially the dust grains lurking in the interstellar medium. When travelling at a good fraction of the speed of light, dust grains become missiles that can inflict terrible damage on a starship. Yet most of the arguments against interstellar travel have a hidden assumption: that starfarers live in an environment like that found around the Solar System. My first paper proposes the idea of galactic traversability — interstellar flight may be much easier in some types of galaxies than others.

Let's take the distance between stars. The flight time depends on the density of stars, but stellar density is not constant even within galaxies. The space around the centers of galaxies is relatively packed with stars, so interstellar flights will be shorter and easier to accomplish. This in fact is one of the motivations for Breakthrough Listen's Galactic Center survey. But there's another way to speed up starflight: rather than trying to go to the stars, one could let the stars come to you. This is faster in some galaxies than others. In large elliptical galaxies, stars are moving every which way at hundreds of kilometers per second. Those speeds are slow compared to those of hypothetical fusion or antimatter rockets, but they're much faster than the speeds we achieve with our chemical rockets. It's all a matter of patience, but in elliptical galaxies, one does not have to be as patient.

Galactic traversability also considers the interstellar medium. The Milky Way is a star-forming galaxy, and that star formation is fueled by large (though declining) reservoirs of gas and the dust within it. But not all galaxies are star forming. The quiescent galaxies include most elliptical and lenticular galaxies. Not all of these galaxies are entirely clear of dust and gas, but most of them have much less than the Milky Way, and the ones in galaxy clusters — like the Virgo ellipticals we are observing — have very little. These galaxies may be "super-traversable": interstellar travel could be much easier than in the Milky Way. What I find most interesting is that these galaxies actually contain the majority of stars in the universe. Perhaps most ETIs do not find flight to nearby stars to be hazardous journeys filled with peril, at least not to the extent we would. The most traversable galaxies in my judgement are the compact elliptical galaxies. These appear to be the former cores of larger ellipticals whose outer envelopes have been ripped away by tidal forces. As the cores of galaxies, their stellar densities are high, the stars move around fast, and they're also thought not have much of an interstellar medium. Although fairly rare, we are fortunate enough to have a compact elliptical in our intergalactic backyard: Messier 32, a satellite of the Andromeda galaxy.

But there's a second argument against starfaring, and it seems to rise from a notion at the heart of the SETI enterprise: the Copernican Principle. The Copernican Principle says that we are not at a special location in the universe. We do not think life and intelligence are miracles that could only appear on our Earth, but evolve through processes with some chance of happening on other worlds, nearly inevitably in a large enough universe. We are sometimes said to live around an unremarkable star in an unremarkable galaxy. But if the Earth is fairly normal, doesn't that mean we should expect ourselves to be fairly typical as well? For example, some astronomers believe this means that we should expect most ETIs to live in similar environments as us, around yellow stars like the Sun instead of the more numerous red dwarfs and in spiral galaxies instead of ellipticals. One of the most controversial takes on the Copernican Principle is the Doomsday Argument, which says that we probably are not living in a special time in our history, but instead we're more likely in the middle. Although that seems innocent enough, it directly implies that interstellar migration will not happen. A long-lived interstellar society with millions of inhabited worlds could sustain countless trillions of inhabitants, but only about 100 billion humans have been born so far. So if humanity is destined to go to the stars, we are among the very first humans to ever live — and the Doomsday Argument rejects such an extreme "position" in history. According to the Doomsday Argument, this is not a mere improbability either: the odds could easily be countless trillions to one against that. Even without interstellar travel, the Doomsday Argument also rules out a long future on Earth, which directly ties into the lifetime factor of Drake's equation. If we accept the argument, detecting ETIs is nearly impossible.

These "typicality" or "Copernican" arguments have high stakes not just in SETI, but in fundamental physics and cosmology. They may even be needed when reasoning about what we see in an infinite universe. My second paper, "The Noonday Argument", is my take on the subject. One theme is that not every plausible-sounding appeal to the Copernican Principle is valid. The title's Noonday Argument is an example: just as we are not at the center of the universe, surely we're not at the center of history? According to the Noonday Argument, the "beginning" of history is one of the most reasonable places we could be. The Noonday Argument does parody one version of the Doomsday Argument, but there is a rather different version based on Bayesian reasoning, which interprets probability as a way to measure our confidence ("credence") in a belief. Basically, according to this version, there are more possible people you could have been in a long interstellar future, and by a kind of Occam's Razor, models with fewer people are favored. One of my arguments is to take this reasoning to the extreme. A favorite topic of philosophy students is solipsism: maybe you're the only real person in existence and you're just dreaming up everyone else! The reasoning behind the Doomsday Argument favors solipsism by a factor of billions to one. You're the most typical person in existence if you're the only person in existence! I contend this actually goes against the spirit of Copernican Principle, because instead of there being many worlds, there would be just one — yours.

The bulk of the paper delves into the philosophy as it tries to balance cases where typicality can work and when it should not. What I suggest is that instead of having just one probability distribution like in normal Bayesian reasoning, you would have several, each one made under the assumption that you are at a particular location. These get averaged together to get our level of belief. As we narrow down the range of locations we could be located at, the averaging changes to reflect this. Another theme of the paper is that details matter. Rather than considering everyone to be an interchangeable "observer", it matters who is who. Typicality emerges from the number of different ways intelligent beings can be arranged in time and space, just as a fair coin will tend to turn up heads half the time if flipped enough times even though a sequence of all-heads is just as likely as any other specific series of coin flips. If my work is right, then the Doomsday Argument fails when we try to apply it to ourselves because of both the averaging and combinatorics. My paper does not solve all the puzzles related to these typicality arguments — there are some situations where the system I suggest tends to favor the existence of "exotic" intelligences — but it adds another piece to the puzzle. My personal guess is that we cannot rule out the existence of intelligences unlike ourselves just because we're humans living on Earth in the year 2021. Instead, the only way to tell is to actually look at the universe out there.

The possible existence of interstellar societies is a game-changer in SETI if true. Interstellar travel can multiply the number of inhabited worlds by a tremendous factor, making it more likely there's at least one at any given time that's interested in communicating. Additionally, it takes just one species from one planet to attain interstellar flight to effectively end up with millions. Even in galaxies where habitable planets may be rare could then be interesting SETI targets. If my papers are on the right track, we should consider a wide variety of galaxy types and search for thse interstellar societies. Fortunately, Breakthrough Listen is doing just this as we observe nearby galaxies of many types and the menagerie of the Exotica catalog.

The new papers "Galactic Traversability: A New Concept for Extragalactic SETI", and "The Noonday Argument: Fine-Graining, Indexicals, and the Nature of Copernican Reasoning" have been submitted for publication.

M32 image: Fabrizio Francione

As we attempt to answer the question, “are we alone in the universe?”, there are many places to search. Although life might not necessarily arise in environments similar to Earth, Earth-like exoplanets are nevertheless good targets to look for extraterrestrial intelligence. The most recent search for these worlds has been led by the Transiting Exoplanet Survey Satellite (TESS), a space telescope specifically designed with the intention of finding Earth-like planets orbiting nearby stars. TESS finds planets by watching for when they cross in front of their host star and block a fraction of that star’s light — events known as transits. The corresponding difference in brightness depends on how large the planet is compared to its star, and for planets like Earth, this is a tiny fraction. Any star for which TESS observes a transit is cataloged in the TESS Targets of Interest (TOI) catalog.

In the case that a planet does host an intelligent civilization that is at least as technologically advanced as ourselves, then they might be detectable from the activity of their radio communication networks — one type of “technological signature”. A collaboration with the TESS mission has enabled Breakthrough Listen (BL) to conduct a targeted search of TOIs for technosignatures. Using the Green Bank Telescope, BL has observed TOIs and collected data with receivers sensitive to signals in the 1-11 GHz frequency range.

What makes searching TOIs so interesting is that they are not only likely planet hosts, but because observing a star with a transiting planet can lead to a higher likelihood of observing technosignatures from that system. In planetary systems, planets typically orbit uniformly around their star in a disk known as the “ecliptic”. A consequence of this is that in order to be able to watch a planet cross in front of its star, we must be viewing that system edge-on so that our line-of-sight is aligned with their ecliptic. What this means is that Earth is aligned with the ecliptic of every star that TESS has observed a transit for. If an extraterrestrial intelligence (ETI) were aware that its presence is most easily detected by those that can watch their transit, and wanted to make their presence known, they might intentionally beam transmissions (“beacons”) along their ecliptic so that whoever is watching their transit might also pick up their beacon.

Artist’s impression (Danielle Futselaar / Breakthrough Listen) of an exoplanet messaging Earth after seeing it transit the Sun. In a reversal of this geometry, a planet might choose to send signals along its own ecliptic, particularly in the direction opposite to its host star. This could attract the attention of anyone who might be monitoring during planetary transits.

In the summer of 2020, I was an intern at the Berkeley SETI Research Center for Breakthrough Listen. For my project, I worked on adding Google Cloud environment compatibility to the existing data processing pipeline. This allowed me to perform the first BL data analysis that utilized cloud-based computing. I conducted a search for technosignatures in 28 BL observations of TOIs, listening for radio communication networks active within the 1-11 GHz frequency range. We found no technosignatures, but this work has enabled us to put one of the most stringent constraints to date on the presence of technosignatures across such a wide range of radio frequencies.

More details about the analysis can be found in this paper, which is accepted for publication in the Astronomical Journal. The data used in this analysis is available on the Breakthrough Listen open data archive by searching for the target names listed in Table 5 of the paper. This analysis made use of the blimpy and turboSETI software, which are also publicly available.

New approaches in the development of radio telescope arrays enable multiple users to receive and process data simultaneously. This architecture presents unique opportunities to conduct commensal ("piggyback") SETI surveys alongside primary science observations. At the MeerKAT radio telescope in South Africa, Breakthrough Listen is conducting such a commensal SETI survey. Both incoherent and coherent beamforming modes (different methods of electronically "steering" the telescope by combining signals from individual antennas) are planned. The incoherent mode will take advantage of the full field of view of MeerKAT, and signals will be localised post-detection. In addition, up to 64 concurrent coherent beams (with higher sensitivity but a smaller field of view) will be formed on objects of interest within the primary field of view.

One of Breakthrough Listen's goals is to observe a million individual nearby stars, a factor 1000 more than our prior surveys. The BL system on MeerKAT is critical to achieving this. However, when conducting a commensal SETI survey, we have no control over where the telescope points - this is set by the primary observer and their observing strategy, in accordance with their own science goals. Therefore, we need a pre-prepared selection of well-characterised stars distributed across the whole sky as visible to MeerKAT. Such a catalog of stars would ensure that every new pointing encompasses stars of interest within the field of view.

The Gaia data releases contain extensive data on several billion stars, for which their distance can be calculated. Thus Gaia data are a natural place to begin when compiling a catalog of stars to draw from during commensal observations with MeerKAT. We derived a subset of nearby stars from Gaia DR2 by applying a series of quality cuts on various parameters (such as parallax, flux and astrometric excess) to produce a well understood catalog of stars with small errors in distance. The chosen metrics result in a stellar sample of about 32 million stars over the entire celestial sphere, of which approximately 26 million are visible to MeerKAT.

Our system at MeerKAT will be able to beamform on and analyse up to 64 stars simultaneously at the maximum expected datarate. Based on our observing predictions (more on these below), the number of stars available within each field of view could be as high as 3000 in rare cases. Therefore, if the primary pointing is too short, we will have to triage stars for observation. Under such circumstances, observing priority will be as follows:

1. Ad-hoc sources of high importance.
2. Unobserved sources from the full sample of 26 million stars (ordered by distance).
3. Unobserved sources from other supplemental catalogs (e.g. exotica).
4. Sources from the full sample which have already been observed, but for < 5 minutes or with an array containing < 58 antennas.
5. Sources from the full sample that have already been observed, but in a different band.
6. Previously observed sources, ordered by distance.

We also estimated observing progress, measured in the number of unique stars observed over time. It is not possible to predict MeerKAT's observing schedule far in advance, since part of the telescope time is allocated to open-time proposals, the calls for which are made at periodic intervals. However, some information on observations planned by MeerKAT's Large Survey Projects is available. Using this information, we simulated observing progress over the coming months and years. Making a conservative estimate of on-sky telescope time and our own processing requirements, we predict that it will take approximately 1 year and 1 month to observe and analyse 1 million nearby stars, as drawn from our catalog.

Two banks of processing nodes will work to analyze data from over 400,000 stars during the first six months of operations (figure from Czech et al. 2021)

Our observing progress simulations predict that the BL survey on MeerKAT will significantly improve on prior surveys, with the potential to become the most comprehensive SETI survey of its type yet conducted. The search for technologically generated signals, especially those which are narrowband with non-zero drift rates indicative of an extraterrestrial origin, will proceed with unprecedented speed, efficiency and sensitivity.

Our full catalog of approximately 32 million stars (5.4 GB) can be downloaded at: https://seti.berkeley.edu/meerkat_db/BL_MeerKAT_target_list_2021.csv.gz

The paper describing the catalog and our observing strategy has been accepted for publication in the Publications of the Astronomical Society of the Pacific. A preprint is available on the arXiv preprint server.

MeerKAT telescope image credit: SARAO

What happens on galactic scales seems far removed from life on Earth. Galactic habitability is the idea that the cosmic environment of a planet can influence — or inhibit — its life. Studies about galactic habitability have usually focused on the Milky Way and similar galaxies, identifying two limiting factors. First, there needs to be a high enough abundance of elements heavier than hydrogen and helium: these are needed to form solid planets on which life as we know it can evolve. Second, giant cosmic explosions like supernovae and gamma-ray bursts can sterilize planets that are too close. Some have argued the combination of the two leads to a "galactic habitable zone": too close to the galactic core and there are too many supernovae, but too far out there is not enough mass in heavy elements to form planets.

But what if we considered galaxies very different from the Milky Way? Large elliptical galaxies contain about half the stars in the Universe. On both respects they should be very good for life: they have plenty of the heavier elements, but they mostly stopped forming stars billions of years and lack most kinds of supernovae and gamma-ray bursts, which result from young, massive stars. In my newest paper, I suggest a third danger factor: mass extinctions when interstellar comets like 'Oumuamua and Comet Borisov hit planets. This is a threat almost unique to elliptical galaxies, where stars move every which way at very high speeds relative to each other — with random velocities up to about ten times faster than in the Milky Way, where stars tend to revolve around the Galactic Center together in an orderly disk. Not only are the stars moving faster, but so must any comets that formed alongside of them bilions of years ago. These comets act like a cosmic shooting gallery in elliptical galaxies, and habitable planets are among the targets. Faster comets mean both the impact rate is higher and that each impact is much more dangerous. Even a small comet could trigger an event as destructive as the KT impact that famously ended the era of the (non-avian) dinosaurs.

In the paper, I made estimates of the rate of impacts with energy comparable to the KT event using simple models of elliptical galaxies. The random motions of stars in elliptical galaxies should lead to several hundred times more extinction events from interstellar comets than on Earth. However, interstellar comets only very rarely hit Earth: virtually all dangerous impacts are actually the result from asteroids and comets within our Solar System. Thus, planets around most stars in elliptical galaxies are "safe", with billions of years between these events.

Earth-size planets around 5–10% of the stellar population, that found in the cores, experience a KT like impact more often than once every 100 million years. At that rate, the collisions could dominate the rate of mass extinctions and maybe prevent the evolution of intelligence. Planets in certain compact elliptical galaxies — in particular, the Andromeda satellite galaxy M32 — may also be in danger from the high density of comets expected there. The hearts of large elliptical galaxies may turn out to be dangerous, but elliptical galaxies as a whole remain worthy targets for SETI and Breakthrough Listen.

The paper has been submitted to Astrophysical Journal Letters, and a preprint is availabe on arXiv.

Messier 49 image: ESA/Hubble & NASA, J. Blakenslee, P. Cote et al.
Asteroid impact artist's impression: NASA / Don Davis
Impact rate figure: Lacki (2021)

FRB 121102 was the first Fast Radio Burst (FRB) source discovered to repeat. Over the course of many observations since its discovery, a significant number of low-energy (or 'smudgy') narrowband bursts have evaded detection in standard searches—many just surpassing telescope detection thresholds. Breakthrough Listen caught a plethora (72, to be exact) of these bursts from FRB 121102 with the Green Bank Telescope (GBT) in 2018. These ubiquitous, yet elusive bursts raise an important question in the realm of FRB detection: what are we missing?

We re-analyzed the 2018 GBT observation of FRB 121102 taken at high frequencies of 4-8 GHz. Our aim was to calibrate and investigate the as-of-yet unmeasured polarization properties of eight of these dim bursts, originally caught by a machine-learning-based transient search method involving convolutional neural networks. All of these lower energy bursts were undetectable with standard full-band search techniques, so to improve our sensitivity, we lowered the detectable signal-to-noise threshold by dividing the observing band into sub-bands and searching each individually. With this simple tweak, we were able to uncover 20 of the 72 bursts, eight of which were bright enough to allow for accurate polarization analysis.

Eight low-energy narrowband bursts from FRB 121102 at C-Band (4-8 GHz). The alignment of the linearly polarized data (dashed red lines) with the total intensity (solid black lines) in the time series indicates that all bursts are nearly 100% linearly polarized. Above that, the polarization position angles (red scatter plots) appear roughly stable (flat) across each burst.

In the end, we confirmed that these bursts showed polarization position angles and Faraday rotation measures (the "twisting" in polarization direction that occurs when radio waves at different frequencies travel through magnetized plasma in the interstellar medium) consistent with other bursts from FRB 121102 that had been detected around the same time. This study, among others, emphasizes the importance of searching for weak bursts in FRB data. Discovering burst-types of varying brightness might very well give rise to new insights into the mechanism behind their emission, or the interstellar material through which they traveled on their long and arduous journey to Earth.

All the data used in this analysis, as well as the polarization plots, are publicly available. An overview of our methods and results has been published in the Research Notes of the American Astronomical Society.

Banner image: Danielle Futselaar / Breakthrough Listen

Now in the eleventh month of observations of exoplanet host stars identified by TESS, Breakthrough Listen continues to collect data from the Green Bank Telescope (GBT) and Parkes Telescope in search for technosignatures. Using exoplanet candidate lists provided by the TESS Team at Massachusetts Institute of Technology (MIT), we have plans to observe all TESS-identified planets in the northern celestial sphere from 1-12 GHz with the GBT, and southern targets with Parkes with their newly available Ultra-Wide Band receiver with simultaneous frequency converge from 0.7-3.8 GHz.

In July 2020, as the TESS mission ended its two-year primary survey, one year centered on the southern ecliptic pole, and the second year centered on the northern, TESS started its extended mission by re-observing the southern sky. At the same time, Breakthrough Listen summer intern Piper Stacey was simulating transiting megastructures and how they might look in TESS data. Analyses such as these are focused on modeling non-circular shapes that are distinctly different from the round shape of transiting exoplanets and they set the stage for searching TESS’s optical lightcurves for technosignatures.

Radio SETI observations of TESS identified exoplanets are ongoing and we now have 585 30-minute cadence observations at Parkes and 1464 observations from GBT with several receivers between 1 and 12 GHz. The GBT observations include 97 exoplanet host stars that have been observed across that entire frequency range. Analysis of these observations with TurboSETI is ongoing with first results coming soon. Summer intern Raffy Traas worked hard to get the initial SETI analysis of these targets from the GBT setup in the Google cloud, laying the groundwork for easier reproducibility and expansion of the code used for detecting narrowband signals.

The SETI w/ TESS Collaboration continues to share ideas about prioritization of TESS-identified planet candidates, and we are currently formalizing a SETI prioritization schema. Stay tuned for more information on TESS-SETI developments.

If you have any questions or would like to join our SETI with TESS monthly meetings, please contact me at hisaacson@berkeley.edu.

Number of observations at each telescope:

GBT
Number of observations at each telescope
GBT: 944 targets in the database
L-band: 149
S-band: 448
C-band: 464
X-band: 403
LSCX-band: 97
Parkes: 679 in the database
UWL(0.7 to 3.8 GHz): 585
(image from: https://www.nasa.gov/image-feature/goddard/2020/nasa-s-tess-creates-a-cosmic-vista-of-the-northern-sky)

Hello! I’m Piper and I’m currently a summer intern at the Berkeley SETI Research Center with Breakthrough Listen. My project for the summer is to look for signs of artificial structures or objects orbiting stars as part of our collaboration with scientists from the TESS mission.

After a dive into the literature, I read a paper by Emily Sandford and David Kipping outlining their work on modeling structures transiting in front of stars. I downloaded their code, EightBitTransit, and then it struck me. What would my name look like if it transited in front of a star? What would the names of the other interns look like?

Now, I recognize that it is highly unlikely that extraterrestrial intelligence would a) use the same lettering system as a fraction of Earth and b) choose to write one of this year’s intern’s names as a megastructure around their host star, but as one of the other interns has mentioned, the resulting light curves would make pretty cool signatures. Additionally, it was a step in the right direction towards modeling more plausible structures:

The Black Box:

The Destroyed Planet:

The Prime Number Signaling:

What’s next? Well, some possible ETI megastructures create much clearer light curves than others as displayed above. So, I’ll be working with the Berkeley SETI Research Center and TESS to sort through TESS data to find the limits of possible existing megastructures within TESS’s observed sources. We hope to use machine learning to sift through data and from this we hope to find an upper limit to the hypothetical presence of ETI megastructures within the data.

Thanks to Sandford and Kipping for making the code that generated these lightcurves publicly available.

Main image credit: Kepler 11 (NASA / Tim Pyle) modified by Steve Croft

We report a detection of Swift J1818.0-1607 across 4 to 11 GHz (C and X-bands) using the Robert C. Byrd Green Bank Telescope (GBT) and the Breakthrough Listen digital backend (MacMahon et al. 2018, PASP, 130, 044502). The Breakthrough Listen Initiative is conducting a wide ranging search for observational indicators of extraterrestrial civilizations (technosignatures), including observations of a variety of exotic objects such as magnetars (Lacki et al 2020 in prep).

On UT 2020 March 12, 21:16:47 the Swift Burst Alert Telescope reported outburst activity from SGR Swift J1818.0-1607 (GCN circular 27373). Following up on this activity, NICER reported a coherent periodic feature with 0.733417(4) Hz periodicity and suggested the source to be an active magnetar. Several radio follow-up campaigns (ATels #13353, #13554, #13559, #13560, #13562, #13569) detected radio pulsation from the source below 3 GHz. Karuppusamy et al. (ATel #13553), reported the first detection at 1.4 GHz at the DM of 706(4) pc-cm^-3 from the Effelsberg radio telescope, with a followup confirmation by Rajwade et al. (ATel #13554) and Champion et al. (ATel 13559) from the Lovell Telescope, and Lower et al. (ATel #13562) from the MeerKAT telescope at similar frequencies. Mann et al. (ATel #13560) reported a 2 GHz detection from the GBT suggesting a spectral index of -1.8(3). Champion et al. (ATel 13559) carried out observations at 6 GHz with 4 GHz of instantaneous bandwidth using the Effelsberg telescope with no detection.

Using the GBT, we carried out a 600 second observation on UT 2020 March 19, 12:22:47 at 6 GHz with 4 GHz instantaneous bandwidth (C-band), immediately followed by another 600 second scan at UT 13:07:57 at 9.3 GHz with 3.7 GHz of instantaneous bandwidth (X-band). Observations were recorded as GUPPI-formatted baseband voltages (Lebofsky et al. 2019, PASP, 131, 124505). Data were processed with a bespoke GPU-accelerated pipeline to convert voltages to high temporal resolution SIGPROC formatted filterbank products, and voltages were preserved. We used the periodicity of 0.7334110 Hz reported by Enoto et al. (ATel #13551) and DM of 703 pc-cm-3 (Rajwade et al. ATel #13554) to produce folded profiles at both bands. Data affected by RFI were flagged using PSRCHIVE tools.

At C-band, we found a prominent profile with integrated S/N of around 16. Based on the radiometer equation we estimate that the approximate peak flux density of the detection is around 0.04 mJy. We only included the lower 2 GHz of C-band for the integrated profile. Upon closer inspection, we noticed that this average profile actually consists of a few intermittent, prominent, and bright bursts instead of regular weaker single pulses (see linked figure). At X-band, we did not find a significant profile. A separate search for single pulses was undertaken for both the C-band and X-band data, using the technique described by Gajjar et al (2018, ApJ, 283, 2).

The preliminary analysis detected 29 single pulses across 4 - 8 GHz with S/N ranging between 6.2 to 146, corresponding to peak flux densities of 21 to 500 mJy.

We also report a marginal detection of a weak burst across 8 - 11 GHz from these observations with a S/N of around 7.2 at the DM of 703 pc-cm^-3. This corresponds to a peak flux density of around 40 mJy.

We report a wide variety of single pulses with multiple sub-pulse components and varying pulse widths (see linked figures), similar to other radio magnetars (Pearlman et al. 2018, ApJ, 886, 17; Maan et al 2019, ApJ, 882, 9; etc). A leading theory for Fast Radio Bursts (FRBs) is that they originate from magnetars; however, most FRB searches have been undertaken at lower frequencies (< 3 GHz). Our detections suggest that many FRBs are likely to be active above 4 GHz, given flatter spectral indices of radio emission from magnetars. Detection of bright single pulses further supports these claims.

Plots for 29 single pulses detected across 4 - 8 GHz

These observations suggest that Swift J1818.0-1607 is still active and we encourage further follow-up observations.

We thank the GBT staff for all their help during these observations.

We are releasing all of our data taken during these observations to the community.

Download SIGPROC filterbank file from C-band observations

Download SIGPROC filterbank file from X-band observations

Baseband data are available here for download

Data are released under the CC BY 4.0 license. If you make use of these datasets for academic work, please cite Gajjar et al. 2020 ATel 13575

If you have any questions, please do write to me at vishalg@berkeley.edu

In SETI, brighter is better: the more powerful a signal is, the further away we could detect it. Maybe aliens use huge power plants to keep a beacon going in hopes of grabbing our attention. Yet the Universe is already filled with brilliant objects, so maybe extraterrestrial intelligences merely need to send the light from those our way. Lenses and mirrors are familiar tools to re-direct light, but they have limitations. A very fundamental one is that lenses and mirrors at best preserve the surface brightness of light, the amount of light coming from each part of their sky. That's why if you put a magnifying glass out in deep space in front of the Sun, astronomers wouldn't notice any difference: the lens would look only as bright as the parts of the Sun it blocked. To do something really noticeable to astronomers on other planets this way, you'd need a magnifying glass as large as the Sun.

Extraterrestrials wanting to use a lens to send a signal flare to us would need to choose something small and bright as a light source, which means something hot. Among the hottest things in the Universe are neutron stars. These are the cores of dead stars once much bigger than the Sun. A single neutron star contains more mass than the Sun and is the size of a city, and has an enormous surface gravity. Because of that gravity, things that fall onto it accelerate to a few percent of the speed of light and release an enormous amount of energy on impact. Some neutron stars orbiting close to a star like our Sun rip gas from their companion with their gravity, sending a continual downpour onto their surfaces. That gas heats to thirty million Kelvin when it hits, hotter than the Sun's core. These systems are called low mass X-ray binaries, and they are some of the brightest things in a galaxy. In some of them, the power of a hundred thousand Suns or more can come from a region the size of a town. The catch: almost all of this light is in X-rays.

The idea of my new paper is that technologically advanced extraterrestrial societies could boost the brightness of these objects even further with an X-ray lens. Suppose they have X-ray lenses that are a thousand kilometers across—certainly huge by our standards, but tiny compared to the Sun—and they place them in orbit around an X-ray binary. When one of the lenses passed directly in front of the neutron star, the system would appear to brighten several thousand times for about one second. These "lens flares" could be as bright as a billion Suns, or a small galaxy. Most remarkably, the extraterrestrial intelligences spend no energy of their own on these flares. All it takes is a properly aimed lens. Of course, there would be a lot of challenges. They would have to travel thousands of light years to a bright X-ray binary. They would need to find material to build the lenses. That's a big problem since a neutron star is born in a supernova that obliterates the planets and asteroids in any solar system it's in, and if anything survived that, the X-ray binary itself would evaporate rock out to beyond the distance Jupiter orbits the Sun. They would need a lot of lenses to make sure astronomers in all directions would see a flare. Then, they would need to make sure the lenses don't crash into each other, or lose their aim on the neutron star. If these problems could be solved, though, the lens flares could act as a beacon lasting thousands, even millions of years.

We would need to look for lens flares in X-rays. X-ray SETI is not at all common, although people suggested that extraterrestrial intelligences might drop asteroids on neutron stars decades ago. If a lens flare happened in the Galaxy, however, it would be among the most powerful X-ray events astronomers have ever seen. On the other hand, they are probably very rare, with decades between each lens flare. We have had X-ray satellites capable of detecting them in the Galaxy for at least twenty years. More common would be shadowing events, when the lens is off-center with the neutron star and magnifying empty space. While they would be nowhere nearly as obvious, they might be a hundred times more common, as the many lenses in orbits tilted slightly with respect to us keep passing over the neutron star. If we happened to be really lucky with the timing, we might even be able to see a lens flares out to about thirty million light years from Earth—an intergalactic beacon requiring no power of its own.

You can read a preprint of my article describing lens flares on the arXiv pre-print server.

Header image credit: NASA

A mysterious stellar system was identified this year, exhibiting 28 transit-like events over 87 days in Kepler data. This system is called EPIC 249706694 / HD 139139, and many proposed astrophysical transit scenarios have failed to properly explain the origin of these transits — giving the system the nickname “the random transiter”. This sounded like an excellent opportunity to search for technosignatures!

We took follow-up observations of HD 139139 at radio frequencies of 4-8 GHz, using the Breakthrough Listen instrument at the Green Bank Telescope. Three observations were made with the telescope pointed at the system and three slightly offset, which allows us to distinguish whether a detected signal is actually coming from the source of interest. Unfortunately, we did not detect any signals of SETI interest, but based on our search sensitivity, we were able to set a limit on the effective power of a potential transmitter to be about 10 TW. In comparison, the most powerful transmitter at the Arecibo Observatory can reach a power of about 20 TW!

All of the observational data used is publicly available online, and a technical summary of our analysis will be published in Research Notes of the American Astronomical Society.

How thinking about changing colors could help us catch signals from extraterrestrial intelligence that would otherwise slip through the cracks

Let’s do a thought experiment.

Imagine you’re out in space with your friend and they shine a green light at you. If your friend starts travelling towards you at a constant speed, that green light will appear a little bluer (called a blueshift) as the wavelengths of light get compressed. If they instead start travelling away from you at a constant speed, the green light will appear a little closer to yellow (called a redshift).

Now let’s take this a little further: imagine your friend starts at a standstill with their green light, and then starts accelerating towards you, moving faster and faster. What will you see? The light will start as green, and shift bluer and bluer as time goes on. This effect is called a Doppler acceleration, and it happens to any electromagnetic wave whose source is accelerating towards or away from you - including radio waves.

But let’s continue this analogy in visible wavelengths of light. Let’s say you’re looking for a flashlight from an extraterrestrial intelligence (ETI). We don’t know what color to expect - red, yellow, blue, purple, who knows what color the ETI have chosen. So we tell our computers* to search for bright spots in every color that our telescope can collect. But we have to be careful - if the color is changing throughout the observation, because of Doppler acceleration, the computer might miss it because it’s just looking for signals of a single color!

Okay, but why would the ETI’s transmitter (the radio equivalent of our flashlight from before) be accelerating in the first place?

Turns out, space is full of things that are constantly accelerating towards and away from us (radial acceleration). A planet orbiting a star is accelerating. A transmitter on the surface of a rotating planet is accelerating. A transmitter that’s orbiting a planet like a satellite is accelerating. Earth is accelerating around its rotation axis and around the sun as we try to take our data, causing the same issues via symmetry!

All of these effects stack, and cause a transmitter that is sending out a single wavelength of light to appear to change drastically over time.

In our new paper, we wanted to calculate exactly how drastically the wavelengths would change over time. We can tell our computers to search for these drifting signals through time, but we have to give them a maximum limit of how much drift to expect. So what’s the maximum?

To answer that question, we considered every planet in our solar system, every known exoplanet, all of the asteroids and comets in our solar system, orbits around main sequence stars, neutron stars, and even black holes. We calculated the fastest acceleration we would expect in each case, and what its expected Doppler drift would be.

In the end, we found that the searches for extraterrestrial intelligence in the past have used a maximum Doppler drift rate that was too small, leading to the potential for missed signals. Luckily, with the new guideline (200 Hz/s at 1 GHz, for those of you who want units) we should be able to catch signals from ETI no matter what the acceleration is like in their home system!

The new paper has been accepted to the Astrophysical Journal and is also available on the arXiv preprint server.

*there’s waaaay too much data for our team to go through it all by eye, so we have to write algorithms to find interesting signals for us

Post by Sofia Sheikh, former Berkeley SETI undergraduate intern - now a PhD student in Astrobiology at Penn State University

Today, we are pleased to announce our release of 1 petabyte of Breakthrough Listen data, and two academic papers as submitted to leading astronomy journals. Building on the results we presented in 2017, we have now submitted a more wide-ranging and detailed analysis of 1327 nearby stars -- 80% of our nearby star sample. These new results represent the most comprehensive and sensitive radio search for extraterrestrial intelligence (SETI) in history.

Further details can be found in our press release, and supporting materials can be found at seti.berkeley.edu/listen2019.